feature/featuretags, all: add ts_omit_acme to disable TLS cert support

I'd started to do this in the earlier ts_omit_server PR but
decided to split it into this separate PR.

Updates #17128

Change-Id: Ief8823a78d1f7bbb79e64a5cab30a7d0a5d6ff4b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -4,9 +4,7 @@
 package ipnlocal
 
 import (
-	"crypto/x509"
 	"encoding/json"
-	"encoding/pem"
 	"errors"
 	"fmt"
 	"io"
@@ -54,9 +52,6 @@ var c2nHandlers = map[methodAndPath]c2nHandler{
 	req("POST /logtail/flush"): handleC2NLogtailFlush,
 	req("POST /sockstats"):     handleC2NSockStats,
 
-	// Check TLS certificate status.
-	req("GET /tls-cert-status"): handleC2NTLSCertStatus,
-
 	// SSH
 	req("/ssh/usernames"): handleC2NSSHUsernames,
 
@@ -497,54 +492,3 @@ func regularFileExists(path string) bool {
 	fi, err := os.Stat(path)
 	return err == nil && fi.Mode().IsRegular()
 }
-
-// handleC2NTLSCertStatus returns info about the last TLS certificate issued for the
-// provided domain. This can be called by the controlplane to clean up DNS TXT
-// records when they're no longer needed by LetsEncrypt.
-//
-// It does not kick off a cert fetch or async refresh. It only reports anything
-// that's already sitting on disk, and only reports metadata about the public
-// cert (stuff that'd be the in CT logs anyway).
-func handleC2NTLSCertStatus(b *LocalBackend, w http.ResponseWriter, r *http.Request) {
-	cs, err := b.getCertStore()
-	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
-		return
-	}
-
-	domain := r.FormValue("domain")
-	if domain == "" {
-		http.Error(w, "no 'domain'", http.StatusBadRequest)
-		return
-	}
-
-	ret := &tailcfg.C2NTLSCertInfo{}
-	pair, err := getCertPEMCached(cs, domain, b.clock.Now())
-	ret.Valid = err == nil
-	if err != nil {
-		ret.Error = err.Error()
-		if errors.Is(err, errCertExpired) {
-			ret.Expired = true
-		} else if errors.Is(err, ipn.ErrStateNotExist) {
-			ret.Missing = true
-			ret.Error = "no certificate"
-		}
-	} else {
-		block, _ := pem.Decode(pair.CertPEM)
-		if block == nil {
-			ret.Error = "invalid PEM"
-			ret.Valid = false
-		} else {
-			cert, err := x509.ParseCertificate(block.Bytes)
-			if err != nil {
-				ret.Error = fmt.Sprintf("invalid certificate: %v", err)
-				ret.Valid = false
-			} else {
-				ret.NotBefore = cert.NotBefore.UTC().Format(time.RFC3339)
-				ret.NotAfter = cert.NotAfter.UTC().Format(time.RFC3339)
-			}
-		}
-	}
-
-	writeJSON(w, ret)
-}